Positive displacement flowmeters typically include oval rotor, rotary vane and gear wheel flowmeters. Oval rotor flowmeters for example comprise a pair of oval shaped rotors as the primary measuring elements. With those meters having direct pulse generation magnets are fitted to the rotors. These rotors rotate in accordance with fluid flow passing through the meter. One or more fixed sensors or signal generating means such as reed switches, hall effect devices, coils or proximity switches etc. are excited to generate an electrical pulse each time one of the rotor mounted magnets passes. An output train or series of such pulses is necessary for the purpose of flow rate integration and calculation.
One disadvantage with such prior art flowmeters is that transient flow reversals adversely affect the quality and point accuracy of the flowmeter output. Such flow reversals occur for example in internal combustion engine fuel supply systems and pulsating flow lines. They can cause substantial inaccuracies in the measurement of fluid flow in the forward direction. More specifically, with prior art devices such as that shown schematically in FIG. 1, magnets 1 are mounted on respective lobes 2 of rotors 3. As the rotors rotate these magnets each pass a single fixed sensor 4, which is thereby activated to generate one pulse. After the magnetic field from each magnet passes out of the sensor activation zone the sensor automatically resets itself for the next pass. If at any time during this process the fluid flow direction and rotor rotation briefly reverses, the last activating magnet after leaving can re-enter the sensor activation zone to cause generation of an additional aberrant pulse. This can produce a point error of as much as 50% to 100%. This condition can apply to a magnet either approaching or leaving the sensor activation zone. Generally the greater the order of reverse flow the larger is the potential for erroneous pulses. These pulses result in inconsistent and inaccurate readings being transmitted to the receiving instrument.